1. Field of the Invention
The present invention relates to a ceramic dielectric composition useful in the manufacture of ceramic capacitors having base metal electrodes capable of being cofired during the sintering of the ceramic.
2. Description of Prior Art
Base metal electrode ceramic capacitors, dielectric compositions from which they are made, and methods for their manufacture are well known in the art. The primary advantage of these capacitors is the elimination of costly noble or other precious metal electrode systems. Typically, base metal electrode capacitors of the monolithic or multilayer type are made with barium titanate-based compositions and are sintered in reducing atmospheres to prevent oxidation of the base metal electrodes. Partial replacements for Ba and Ti, for example Ca and Sr for Ba, and Zr and Hf for Ti, are used to enhance the dielectric properties of the capacitors and/or to shift the dielectric (Curie) peak to normal operating temperatures. The replacements typically have the same valency as the atom replaced. It is also known that sintering these compositions in atmospheres of low oxygen partial pressure tends to render the ceramic semiconductive and, to inhibit the detrimental formation of a semiconductive body, suitable acceptor dopants are also added to the ceramic compositions. Typical of dopants used in the art are Mg, Al and Ga, and certain transition metals such as Cr, Mn, Fe, Co and Ni.
The addition of acceptor dopants, however, often results in at least two other problems, both affecting the Curie peak position of the sintered material. One problem is the need to carefully control the amount of the dopant added, particularly at low concentration levels, and the other concerns the generally observed broad change in the Curie peak temperature with changes in the oxygen partial pressure of the atmosphere in which the ceramic is sintered.
U.S. Pat. Nos. 3,920,781, 4,089,813, 4,115,493 and 3,987,347 are representative of the prior art teachings of compositions and methods for making base metal electrode ceramic capacitors. U.S. Pat. No. 3,920,781 discloses the addition of acceptor dopants in a barium titanate composition and a careful additional cation balancing to maintain the large to small cation ratio between 0.95 and 1.0 (large and small cations being defined, respectively, as those having ionic radii greater and less than 0.90 Angstrom unit). Although good results in the form of capacitors exhibiting suitable dielectric properties are reported, this patent teaches the necessity of maintaining absolute control over the reducing atmosphere in which the capacitors are fired to prevent dissolution and degradation of the base metal (nickel) electrodes. Further, with a decrease in the oxygen partial pressure of two orders of magnitude from the disclosed maximum of 10.sup.-7 atmosphere, U.S. Pat. No. 3,920,781 reports an unacceptably large increase of about 50.degree. C. in the Curie peak temperature.
U.S. Pat. No. 4,089,813 also discloses a barium titanate based composition to which are added various mixed or binary oxides. Also included is a substantial addition of barium oxide such that the large to small cation ratios of the compositions in the ranges disclosed are maintained at or slightly above 1.00. The compositions disclosed in this patent are suitable for the manufacture of ceramic dielectrics with relatively low permittivities (dielectric constants up to about 2200), but would be unsuitable for the manufacture of high K monolithic capacitors.
U.S. Pat. No. 4,115,493 discloses a range of compositions based on barium titanate with isovalent substitutions of calcium and zirconium and to which an excess of Ba and Ca is added, to maintain a large to small cation ratio (L/S) of from 1.005 to 1.03. Although no acceptor dopants are intentionally added to the disclosed compositions, the stated purities of the starting compounds suggest the presence of impurities at levels up to 1%. It is well known, as illustrated in their Example 3, that impurities at such concentrations can have a significant effect on the ultimate dielectric properties of a ceramic capacitor.
U.S. Pat. No. 3,987,347 teaches the importance of close control of acceptor dopant concentrations in ceramic compositions for base metal electrode capacitors. This patent teaches that it is essential to limit the acceptor dopants to very small quantities, the total of which should not exceed 2.5 mole %. The range of compositions disclosed is balanced to maintain a large to small cation ratio of 0.975 to 1.00. The compositions disclosed in this patent are suitable for the manufacture of ceramic dielectrics with relatively low permittivities (dielectric constants up to about 3000), but would be unsuitable for the manufacture of high K monolithic capacitors.